ABSTRACTA mathematical process model is introduced, simulating the deposition of suspended material in lakes and reservoirs. The model is applied on the deposition processes in a reservoir within the Reventazon river basin in Costa Rica, Central America. The calculations show a trap efficiency of the reservoir of about 80 per cent. The influence is quantified of variations in grain size composition of the suspended material and of water discharge changes on the resulting grain size composition of bottom material in different parts of the reservoir. The significance of turbidity currents is elucidated. The methods seem to be useful for the interpretation of palaeohydrological changes with lake sediments as indicators.